Werk #8330: mknotifyd: now supports concurrent delivery of notifications and passes completion results back to the core

The notification spooler will now use multiple queues of notifications, at least one per notification plugin type. This means that, for example, a slow sms notification can not stall delivery of mail notifications. Via global settings it is also possible to increase the number of concurrent messages being sent per plugin type, but caution is advised as we can't guarantee that the specific notification script is actually capable of handling parallel messages.

Another change is that output of the notification scripts will be passed back through the notification daemon to the history log of the core. Ideally this means the "Host- and Service notifications" view will include confirmation that a notification has actually been delivered or an error if it wasn't. We also plan to add a more visible warning when notifications are failing.

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